Method of forming nitride based semiconductor layer

ABSTRACT

A GaN layer is grown on a sapphire substrate, an SiO 2  film is formed on the GaN layer, and a GaN semiconductor layer including an MQW active layer is then grown on the GaN layer and the SiO 2  film using epitaxial lateral overgrowth. The GaN based semiconductor layer is removed by etching except in a region on the SiO 2  film, and a p electrode is then formed on the top surface of the GaN based semiconductor layer on the SiO 2  film, to join the p electrode on the GaN based semiconductor layer to an ohmic electrode on a GaAs substrate. An n electrode is formed on the top surface of the GaN based semiconductor layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device having acompound semiconductor layer composed of BN (boron nitride), GaN(gallium nitride), AlN (aluminum nitride) or InN (indium nitride) or agroup III-V nitride compound semiconductor (hereinafter referred to as anitride based semiconductor) which is their mixed crystal and a methodof fabricating the same, and a method of forming a nitride basedsemiconductor layer.

2. Description of the Background Art

In recent years, a GaN based light emitting semiconductor device (alight emitting semiconductor device based on GaN) has been put topractical use as a light emitting semiconductor device such as a lightemitting diode which emits light in blue or violet or a semiconductorlaser device. In fabricating the GaN based light emitting semiconductordevice, there exists no substrate composed of GaN. Therefore, each layeris epitaxially grown on an insulating substrate composed of sapphire(Al₂O₃) or the like.

FIG. 18 is a cross-sectional view showing the structure of aconventional GaN based light emitting diode. A light emitting diodeshown in FIG. 18 is disclosed in Nikkei Micro Device, February, 1994,pp. 92 to 93.

In FIG. 18, a GaN buffer layer 62, an n-GaN layer 63, an n-AlGaNcladding layer 64, an InGaN active layer 65, a p-AlGaN cladding layer66, and a p-GaN layer 67 are formed in this order on a sapphiresubstrate 61. A partial region from the p-GaN layer 67 to the n-GaNlayer 63 is removed by etching. A p electrode 68 is formed on the topsurface of the p-GaN layer 67, and an n electrode 69 is formed on theexposed top surface of the n-GaN layer 63. Such a structure of the lightemitting diode is referred to as a lateral structure.

The light emitting diode shown in FIG. 18 has a pn junction having adouble hetero structure in which the InGaN active layer 65 is interposedbetween the n-AlGaN cladding layer 64 and the p-AlGaN cladding layer 66,and can emit light in blue.

In a conventional GaN based light emitting semiconductor device as shownin FIG. 18, however, dislocations of around 10⁹/cm² generally exist in aGaN based semiconductor crystal which is grown on a sapphire substratedepending on the difference in lattice constant between GaN and thesapphire substrate. Such dislocations are propagated from the surface ofthe sapphire substrate to a GaN based semiconductor layer. In the lightemitting semiconductor device composed of the GaN based semiconductorlayer on the sapphire substrate, device characteristics and reliabilityare degraded due to the dislocations.

As a method of solving the problem of the degradation of the devicecharacteristics and the reliability due to the dislocations, epitaxiallateral overgrowth has been proposed. The epitaxial lateral overgrowthis reported in “Proceedings of The Second International Conference onNitride Semiconductors”, Oct. 27-31, 1997, Tokushima, Japan, pp.444-446.FIG. 19 is a schematic sectional view showing the steps for explainingthe conventional epitaxial lateral overgrowth.

As shown in FIG. 19(a), an AlGaN buffer layer 82 is grown on a sapphiresubstrate 81, and a GaN layer 83 is formed on the AlGaN buffer layer 82.Dislocations 91 extending in the vertical direction exist in the GaNlayer 83. Striped SiO₂ films 90 are formed on the GaN layer 83.

As shown in FIG. 19(b), a GaN layer 84 is regrown on the GaN layer 83exposed between the striped SiO₂ films 90. Also in this case, thedislocations 91 extend in the vertical direction in the regrown GaNlayer 84.

As shown in FIG. 19(c), when the GaN layer 84 is further grown, the GaNlayer 84 is also grown in the lateral direction. Accordingly, the GaNlayer 84 is also formed on the SiO₂ films 90. No dislocations exist inthe GaN layer 84 on the SiO₂ films 90.

As shown in FIG. 19(d), when the GaN layer 84 is further grown, the GaNfilm 84 is formed on the SiO₂ films 90 and on the GaN layer 83 betweenthe SiO₂ films 90.

When the epitaxial lateral overgrowth is used, a GaN crystal of highquality having no dislocations can be formed on the SiO₂ films 90.

In a region where the SiO₂ films 90 do not exist, however, thedislocations 91 from the underlying GaN layer 83 extend to the surfaceof the regrown GaN layer 84. Accordingly, the dislocations still existon the surface of the GaN layer 84. When the light emittingsemiconductor device is fabricated, therefore, a light emitting regionmust be limited to a region on the SiO₂ films. Therefore, it isimpossible to increase the size of the light emitting region.

When the area of the SiO₂ film is increased in order to increase thearea of the GaN layer of high quality, the surface of the GaN layerwhich is grown in the lateral direction cannot be flattened.

In the conventional GaN based light emitting diode shown in FIG. 18, thesapphire substrate 61 is an insulating substrate. Therefore, the nelectrode 69 cannot be provided on the reverse surface of the sapphiresubstrate 61, and must be provided on the exposed surface of the n-GaNlayer 63. Therefore, a current path between the p electrode 68 and the nelectrode 69 is longer, so that an operation voltage is higher, ascompared with those in a case where the n electrode is provided on thereverse surface of a conductive substrate.

Furthermore, when a GaN based semiconductor laser device is fabricated,it is difficult to form cavity facets by a cleavage method as in asemiconductor laser device for emitting red light or infrared lightusing a GaAs substrate.

FIG. 20 is a diagram showing the relationship between the crystalorientations of a sapphire substrate and a GaN based semiconductorlayer. In FIG. 20, an arrow by a solid line indicates the crystalorientation of the sapphire substrate, and an arrow by a broken lineindicates the crystal orientation of the GaN based semiconductor layer.

As shown in FIG. 20, the a-axis and the b-axis of the GaN basedsemiconductor layer formed on the sapphire substrate are shifted 30°away from the a-axis and the baxis of the sapphire substrate.

FIG. 21 is a schematic perspective view of a semiconductor laser devicecomposed of a GaN based semiconductor layer formed on a sapphiresubstrate.

In FIG. 21, a GaN based semiconductor layer 70 is formed on a (0001)plane of a sapphire substrate 61. A striped current injection region 71is parallel to a <11{overscore (2)}0> direction of the GaN basedsemiconductor layer 70. In this case, a {1 {overscore (1)}00} plane ofthe GaN based semiconductor layer 70 is inclined at 30° to a{1{overscore (1)}00} plane of the sapphire substrate 61. Both thesapphire substrate 61 and the GaN based semiconductor layer 70 areeasily cleaved along the {1{overscore (1)}00} plane.

The respective cleavage directions of the sapphire substrate 61 and theGaN based semiconductor layer 70 thus deviate. When a GaN basedsemiconductor laser device is fabricated, therefore, it is difficult toform cavity facets by a cleavage method, as in the semiconductor laserdevice for emitting red light or infrared light which is formed on theGaAs substrate. In this case, the necessity of forming the cavity facetsby etching is brought about. When the cavity facets are formed byetching, however, it is impossible to reduce an operation current of thesemiconductor laser device because it is difficult to form facetsperpendicular to the substrate.

On the other hand, various reports and proposals are made with respectto methods of controlling a transverse mode of the GaN basedsemiconductor laser device. Almost all of the methods of controlling thetransverse mode include two types, i.e., a ridge waveguided structureand a self-aligned structure which are employed by the conventionalsemiconductor laser device for emitting red light or infrared light.

Since the GaN based semiconductor layer is chemically stable, however,it cannot be patterned by wet etching, unlike an AlGaAs basedsemiconductor layer used for the conventional semiconductor laser deviceemitting red light or infrared light, and must be patterned by dryetching such as RIE (Reactive Ion Etching) or RIBE (Reactive Ion BeamEtching).

In the GaN based semiconductor laser device, therefore, the patterningfor fabricating the ridge waveguided structure or the self-alignedstructure cannot be performed easily and with good reproducibility.Moreover, device characteristics greatly vary depending on the precisionof the dry etching.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a nitride basedsemiconductor device (a semiconductor device based on a nitride) capableof performing a low-voltage operation.

Another object of the present invention is to provide a nitride basedlight emitting semiconductor device capable of performing a low-voltageoperation and forming a facet by cleavage.

Still another object of the present invention is to provide a method offabricating a nitride based semiconductor device in which the number ofdislocations is reduced, a low-voltage operation can be performed, and afacet can be formed by cleavage.

Still another object of the present invention is to provide a method offorming a nitride based semiconductor layer (a semiconductor layer basedon a nitride) of high quality having no dislocations in a wide region ona substrate.

Still another object of the present invention is to provide asemiconductor device composed of a nitride based semiconductor layer ofhigh quality having no dislocations in a wide region on a substrate.

Still another object of the present invention is to provide a lightemitting semiconductor device having an active layer of high qualityhaving no dislocations in a wide region on a substrate.

Still another object of the present invention is to provide a nitridebased semiconductor device capable of fabricating an active region ofthe device of high quality without using etching.

Still another object of the present invention is to provide a refractiveindex guided nitride based light emitting semiconductor device capableof forming an optical waveguide path of high quality without usingetching.

Still another object of the present invention is to provide a method offabricating a nitride based semiconductor device capable of fabricatingan active region of the device of high quality without using etching.

A semiconductor device according to one aspect of the present inventioncomprises a gallium arsenide substrate, a first electrode layer formedon the gallium arsenide substrate, a nitride based semiconductor layerformed on the first electrode layer and containing at least one ofboron, gallium, aluminum and indium, and a second electrode layer formedon the nitride based semiconductor layer.

In the semiconductor device, the first electrode layer and the secondelectrode layer are respectively provided on the bottom surface and thetop surface of the nitride based semiconductor layer on the galliumarsenide substrate. Accordingly, a current path between the firstelectrode layer and the second electrode layer is shortened.Consequently, it is possible to reduce an operation voltage of thenitride based semiconductor device.

The nitride based semiconductor layer may include an active layer. Inthis case, the first electrode layer and the second electrode layer arerespectively provided on the bottom surface and the top surface of thenitride based semiconductor layer on the gallium arsenide substrate.Accordingly, the current path between the first electrode layer and thesecond electrode layer is shortened. Consequently, it is possible toreduce the operation voltage of the nitride based light emittingsemiconductor device.

Particularly, it is preferable that the nitride based semiconductorlayer has a striped current injection region for injecting a currentinto the active layer, the striped current injection region is formedalong a <1{overscore (1)}00> direction of the nitride basedsemiconductor layer, the nitride based semiconductor layer is arrangedon the gallium arsenide substrate such that the <1{overscore (1)}00>direction of the nitride based semiconductor layer coincides with a<110>direction or a <1{overscore (1)}0> direction of the galliumarsenide substrate, and a pair of cavity facets is formed of a {110}plane or a {1{overscore (1)}0} plane of the gallium arsenide substrateand a {1{overscore (1)}00} plane of the nitride based semiconductorlayer.

In this case, the cleavage direction of the nitride based semiconductorlayer and the cleavage direction of the gallium arsenide substratecoincide with each other. Accordingly, the cavity facets can be formedby the cleavage. Therefore, a semiconductor laser device is realized asa light emitting semiconductor device. Further, the uniformity of devicecharacteristics is improved, and the reproducibility of the devicecharacteristics is increased.

A method of fabricating a semiconductor device according to anotheraspect of the present invention comprises the steps of forming a firstnitride based semiconductor layer containing at least one of boron,gallium, aluminum and indium on an insulating substrate, forming aninsulating film in a predetermined region on the first nitride basedsemiconductor layer, forming a second nitride based semiconductor layercontaining at least one of boron, gallium, aluminum and indium usingepitaxial lateral overgrowth on the first nitride based semiconductorlayer and the insulating film, removing the second nitride basedsemiconductor layer except in a region on the insulating film, joiningthe top surface of the second nitride based semiconductor layer on theinsulating film to one surface of a gallium arsenide substrate through afirst electrode layer, removing the insulating film, to remove theinsulating substrate and the first nitride based semiconductor layerfrom the second nitride based semiconductor layer, and forming a secondelectrode layer on the second nitride based semiconductor layer.

In the method of fabricating the semiconductor device, the secondnitride based semiconductor layer is formed using the epitaxial lateralovergrowth on the insulating film on the first nitride basedsemiconductor layer. Accordingly, no dislocations are propagated to thesecond nitride based semiconductor layer on the insulating film from thefirst nitride based semiconductor layer. Consequently, a second nitridebased semiconductor layer of high quality having few dislocations isobtained.

The first electrode layer and the second electrode layer arerespectively provided on the bottom surface and the top surface of thesecond nitride based semiconductor layer on the gallium arsenidesubstrate. Accordingly, a current path between the first electrode layerand the second electrode layer is shortened. Consequently, an operationvoltage can be reduced.

Furthermore, when the top surface of the second nitride basedsemiconductor layer is joined to one surface of the gallium arsenidesubstrate through the first electrode layer, the crystal orientations ofthe second nitride based semiconductor layer and the gallium arsenidesubstrate can be matched with each other. Accordingly, a facet of thesemiconductor device can be formed by cleavage. Consequently, theuniformity of device characteristics is improved, and thereproducibility of the device characteristics is increased.

Particularly, the step of forming the second nitride based semiconductorlayer may comprise the step of forming an active layer. Consequently, alight emitting semiconductor device is fabricated as the semiconductordevice.

Furthermore, the step of forming the second nitride based semiconductorlayer may further comprise the step of forming a striped currentinjection region for injecting a current into the active layer along a<1{overscore (1)}00> direction of the second nitride based semiconductorlayer, and the step of joining the top surface of the second nitridebased semiconductor layer to one surface of the gallium arsenidesubstrate through the first electrode layer may comprise the step ofmatching the <1{overscore (1)}00> direction of the second nitride basedsemiconductor layer with a <110> direction or a <1{overscore (1)}0>direction of the gallium arsenide substrate. The fabricating method mayfurther comprise the step of forming a pair of cavity facets by cleavagealong a {110} plane or a {1{overscore (1)}0} plane of the galliumarsenide substrate and a {1{overscore (1)}00} plane of the secondnitride based semiconductor layer.

In this case, the cleavage direction of the nitride based semiconductorlayer and the cleavage direction of the gallium arsenide substratecoincide with each other. Accordingly, the cavity facets can be formedby the cleavage. Consequently, a semiconductor laser device is realizedas the light emitting semiconductor device. Further, the devicecharacteristics hardly vary, and the reproducibility of the devicecharacteristics is increased.

A method of forming a nitride based semiconductor layer according tostill another aspect of the present invention comprises the steps offorming a first nitride based semiconductor layer containing at leastone of boron, gallium, aluminum and indium on an insulating substrate,forming an irregular pattern including a recess having a bottom surfaceformed of an insulator and a projection having a top surface formed ofan insulator in the surface of the first nitride based semiconductorlayer, and forming a second nitride based semiconductor layer containingat least one of boron, gallium, aluminum and indium on the insulators bygrowth from the first nitride based semiconductor layer using epitaxiallateral overgrowth.

In the method of forming the nitride based semiconductor layer, theinsulators are formed on the bottom surface of the recess in theirregular pattern formed on the surface of the first nitride basedsemiconductor layer and the top surface of the projection in theirregular pattern. Accordingly, the first nitride based semiconductorlayer is exposed to only side surfaces of the irregular pattern.Therefore, the second nitride based semiconductor layer is formed on theinsulators by the growth in the lateral direction of the nitride basedsemiconductor layer. Consequently, no dislocations are propagated to thesecond nitride based semiconductor layer from the first nitride basedsemiconductor layer. As a result, a nitride based semiconductor layer ofhigh quality having no dislocations is formed in a wide region on thesubstrate. The step of forming the irregular pattern may comprise thesteps of forming the irregular pattern such that the first nitride basedsemiconductor layer is exposed to the bottom surface of the recess, andforming an insulating film as the insulator on the bottom surface of therecess of the irregular pattern and forming an insulating film as theinsulator on the top surface of the projection of the irregular pattern.

Alternatively, the step of forming the irregular pattern may comprisethe step of forming an insulating film as the insulator in a region onthe first nitride based semiconductor layer where the projection of theirregular pattern is to be formed and removing the first nitride basedsemiconductor such that the insulating substrate is exposed as theinsulator except in a region on the insulating film.

Particularly, it is preferable that the irregular pattern has a stripedrecess and a striped projection which extend along a <11{overscore(2)}0> direction of the first nitride based semiconductor layer.Consequently, the nitride based semiconductor layer is easily grown inthe lateral direction.

It is preferable that a cross-sectional shape of the projection of theirregular pattern is a rectangular shape or a reversed mesa shape havingvertical side surfaces. When the insulating films are deposited on theirregular pattern, therefore, the insulating films can be formed only onthe bottom surface of the recess in the irregular pattern and the topsurface of the projection in the irregular pattern. Therefore, it ispossible to omit the step of removing the insulating films on the sidesurfaces of the irregular pattern.

A semiconductor device according to still another aspect of the presentinvention comprises an insulating substrate, a first nitride basedsemiconductor layer formed on the insulating substrate and containing atleast one of boron, gallium, aluminum and indium, an irregular patternbeing formed in the surface of the first nitride based semiconductorlayer, insulating films respectively formed on the bottom surface of arecess and the top surface of a projection of the irregular pattern ofthe first nitride based semiconductor layer, and a second nitride basedsemiconductor layer formed on the insulating films and containing atleast one of boron, gallium, aluminum and indium.

In the semiconductor device, the insulating films are respectivelyformed on the bottom surface of the recess in the irregular pattern onthe surface of the first nitride based semiconductor layer and the topsurface of the projection in the irregular pattern, and the secondnitride based semiconductor layer is formed on the insulating films.Accordingly, no dislocations are propagated to the second nitride basedsemiconductor layer from the first nitride based semiconductor layer.Consequently, a nitride based semiconductor device of high qualityhaving no dislocations is realized in a wide region on the substrate.

The second nitride based semiconductor layer may include an activelayer. In this case, the insulating films are respectively formed on thebottom surface of the recess in the irregular pattern on the surface ofthe first nitride based semiconductor layer and the top surface of theprojection in the irregular pattern, and the second nitride basedsemiconductor layer is formed on the insulating films. Accordingly, nodislocations are propagated to the second nitride based semiconductorlayer from the first nitride based semiconductor layer. Consequently, anitride based semiconductor device including an active layer of highquality having no dislocations is realized in the wide region on thesubstrate.

A semiconductor device according to still another aspect of the presentinvention comprises an insulating substrate, a first nitride basedsemiconductor layer formed on the insulating substrate and containing atleast one of boron, gallium, aluminum and indium, a plurality of stripedinsulating films formed a predetermined distance away from each other onthe first nitride based semiconductor layer, and a second nitride basedsemiconductor layer formed on the first nitride based semiconductorlayer and the plurality of striped insulating films and containing atleast one of boron, gallium, aluminum and indium. The second nitridebased semiconductor layer includes an active region of the device abovethe plurality of striped insulating films. The active region is anactually operating region other than an electrode forming region of thesemiconductor device.

In the semiconductor device, the second nitride based semiconductorlayer is formed on the first nitride based semiconductor layer throughthe plurality of striped insulating films. Accordingly, few dislocationsexist in the second nitride based semiconductor layer on a region wherethe plurality of striped insulating films exist. Consequently, theactive region of the device above the plurality of striped insulatingfilms is increased in quality.

On the region where the plurality of striped insulating films exist, thegrowth rate of the second nitride based semiconductor layer is lower, ascompared with that on the region where the plurality of stripedinsulating films do not exist. Consequently, the second nitride basedsemiconductor layer on the region where the plurality of stripedinsulating films exist is concavely formed. Consequently, it is possibleto fabricate the active region of the device without using etching.

The active region may include a light emitting portion. In this case,the second nitride based semiconductor layer is formed on the firstnitride based semiconductor layer through the plurality of stripedinsulating films. Accordingly, few dislocations exist in the secondnitride based semiconductor layer on the region where the plurality ofstriped insulating films exist. Consequently, the light emitting portionabove the plurality of striped insulating films is increased in quality.

On the region where the plurality of striped insulating films exist, thegrowth rate of the second nitride based semiconductor layer is lower, ascompared with that on the region where the plurality of stripedinsulating films do not exist. Consequently, the second nitride basedsemiconductor layer on the region where the plurality of stripedinsulating films exist is concavely formed. Consequently, it is possibleto fabricate a refractive index guided optical waveguide path withoutusing etching.

Particularly, the second nitride based semiconductor layer may have astriped current injection region for injecting a current into the lightemitting portion, and the second nitride based semiconductor layer maycomprise a pair of cavity facets perpendicular to the striped currentinjection region. Consequently, the current is injected in a stripeshape into the light emitting portion, thereby making lasing possible.

It is preferable that the plurality of striped insulating films areformed along a <11{overscore (2)}0> direction of the first nitride basedsemiconductor layer. Consequently, the nitride based semiconductor layeris easily grown in the lateral direction, and the second nitride basedsemiconductor layer is made higher in quality.

A method of fabricating a semiconductor device according to stillanother aspect of the present invention comprises the steps of forming afirst nitride based semiconductor layer containing at least one ofboron, gallium, aluminum and indium on an insulating substrate, forminga plurality of striped insulating films a predetermined distance awayfrom each other on the first nitride based semiconductor layer, andforming a second nitride based semiconductor layer containing at leastone of boron, gallium, aluminum and indium using epitaxial lateralovergrowth on the first nitride based semiconductor layer and theplurality of striped insulating films, to form an active region of thedevice above the plurality of striped insulating films. The activeregion is an actually operating region other than an electrode formingregion of the semiconductor device.

In the method of fabricating the semiconductor device, the secondnitride based semiconductor layer is formed using epitaxial lateralovergrowth through the plurality of striped insulating films on thefirst nitride based semiconductor layer. Accordingly, few dislocationsexist in the second nitride based semiconductor layer on the regionwhere the plurality of striped insulating films exist. Consequently, theactive region of the device above the plurality of striped insulatingfilms is increased in quality.

On the region where the plurality of striped insulating films exist, thegrowth rate of the second nitride based semiconductor layer is lower, ascompared with that on the region where the plurality of stripedinsulating films do not exist. Consequently, the second nitride basedsemiconductor layer on the region where the plurality of stripedinsulating films exist is concavely formed. Consequently, it is possibleto fabricate an active layer of the device without using etching.

A method of forming a nitride based semiconductor layer according tostill another aspect of the present invention comprises the steps offorming a first nitride based semiconductor layer containing at leastone of boron, gallium, aluminum and indium on an insulating substrate,forming an irregular pattern having exposed side surfaces in the firstnitride based semiconductor layer, and forming a second nitride basedsemiconductor layer containing at least one of boron, gallium, aluminumand indium on the irregular pattern by growth from the exposed sidesurfaces of the irregular pattern on the first nitride basedsemiconductor layer using epitaxial lateral overgrowth.

In the method of forming the nitride based semiconductor layer, the sidesurfaces of the irregular pattern formed in the first nitride basedsemiconductor layer are exposed. Therefore, the second nitride basedsemiconductor layer is formed on the irregular pattern by growth in thelateral direction of the nitride based semiconductor layer from theexposed side surfaces. Consequently, no dislocations are propagated tothe second nitride based semiconductor layer from the first nitridebased semiconductor layer. As a result, a nitride based semiconductorlayer of high quality having no dislocations is formed in a wide regionon the substrate.

The step of forming the irregular pattern may comprise the steps offorming the irregular pattern such that the first nitride basedsemiconductor layer is exposed to the bottom surface of a recess, andforming insulating films on the bottom surface of the recess of theirregular pattern and the top surface of the projection of the irregularpattern.

Alternatively, the step of forming the irregular pattern may comprisethe step of forming an insulating film in a region on the first nitridebased semiconductor layer where the projection in the irregular patternis to be formed, and removing the first nitride based semiconductor suchthat the insulating substrate is exposed except in a region on theinsulating film.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view showing the steps of a method offabricating a semiconductor laser device in a first embodiment of thepresent invention;

FIG. 2 is a schematic sectional view showing the steps of the method offabricating the semiconductor laser device in the first embodiment ofthe present invention;

FIG. 3 is a schematic sectional view showing the steps of the method offabricating the semiconductor laser device in the first embodiment ofthe present invention;

FIG. 4 is a schematic sectional view showing the steps of the method offabricating the semiconductor laser device in the first embodiment ofthe present invention;

FIG. 5 is a schematic sectional view showing the steps of the method offabricating the semiconductor laser device in the first embodiment ofthe present invention;

FIG. 6 is a schematic sectional view showing the steps of the method offabricating the semiconductor laser device in the first embodiment ofthe present invention;

FIG. 7 is a diagram showing an energy band structure of an MQW activelayer in the semiconductor laser device shown in FIGS. 1 to 6;

FIG. 8 is a perspective view showing the relationship between thecrystal orientations of a sapphire substrate and a GaN basedsemiconductor layer in the semiconductor laser device shown in FIGS. 1to 6;

FIG. 9 is a schematic sectional view showing the steps of a method offorming a GaN based semiconductor layer in a second embodiment of thepresent invention;

FIG. 10 is a schematic sectional view showing the steps of the method offorming the GaN based semiconductor layer in the second embodiment ofthe present invention;

FIG. 11 is a schematic sectional view showing an irregular pattern in areversed mesa shape formed on the surface of a GaN layer;

FIG. 12 is a schematic sectional view showing a method of forming theirregular pattern in a reversed mesa shape on the surface of the GaNlayer;

FIG. 13 is a schematic sectional view showing an example of asemiconductor laser device fabricated on the GaN layer formed by themethod shown in FIGS. 9 and 10;

FIG. 14 is a schematic sectional view showing the steps in anotherexample of a method of forming a GaN based semiconductor layer;

FIG. 15 is a schematic sectional view showing the steps in anotherexample of the method of forming the GaN based semiconductor layer;

FIG. 16 is a schematic sectional view showing the steps of a method offabricating a semiconductor laser device in a third embodiment of thepresent invention;

FIG. 17 is a schematic sectional view showing the steps of the method offabricating the semiconductor laser device in the third embodiment ofthe present invention;

FIG. 18 is a schematic sectional view of a conventional GaN based lightemitting diode;

FIG. 19 is a schematic sectional view showing the steps of aconventional method of forming a GaN based semiconductor layer usingepitaxial lateral overgrowth;

FIG. 20 is a diagram showing the relationship between the crystalorientations of a sapphire substrate and a GaN based semiconductor layerformed thereon; and

FIG. 21 is a diagram showing the relationship between the crystalorientations of the sapphire substrate and the GaN based semiconductorlayer formed thereon.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 to 6 are schematic sectional views showing the steps of a methodof fabricating a semiconductor laser device in a first embodiment of thepresent invention.

As shown in FIG. 1, an AlGaN buffer layer 2 is first formed on asapphire substrate 1, and an undoped GaN layer 3 is grown on the AlGaNbuffer layer 2. An SiO₂ film 4 having a predetermined width is formed onthe GaN layer 3, and an n-GaN layer 5 is then grown on the GaN layer 3and the SiO₂ film 4 using epitaxial lateral overgrowth.

An n-In_(p)Ga_(1−p)N (P=0.1) crack preventing layer 6 having a thicknessof 0.1 μm, an n-Al_(y)Ga_(1−y)N (Y=0.07) cladding layer 7 having athickness of 1.0 μm, a multi quantum well active layer (hereinafterreferred to as an MQW active layer) 8 described later, ap-Al_(y)Ga_(1−y)N (Y=0.07) cladding layer 9 having a thickness of 0.15μm, and an n-Al_(z)Ga_(1−z)N (Z=0.12) current blocking layer 10 having athickness of 0.20 μm are then formed in this order on the n-GaN layer 5.

FIG. 7 is a diagram showing an energy band structure of the MQW activelayer 8. As shown in FIG. 7, the MQW active layer 8 includes a multiquantum well structure obtained by alternately laminating six InxGalxN(X=0.03) quantum barrier layers 81 having a thickness of 60 Å and fiveIn_(x)Ga_(1−x)N (X=0.10) quantum well layers 82 having a thickness of 30Å. Both surfaces of the multi quantum well structure are interposedbetween GaN optical guide layers 83 having a thickness of 0.1 μm.

A striped region having a width W0 at the center of then-Al_(z)Ga_(1−z)N current blocking layer 10 is then removed by etching.In this case, the striped region between the n-Al_(z)Ga_(1−z)N currentblocking layers 10 is a current injection region 19. The width W0 of thecurrent injection region 19 is 2 μm, for example. The current injectionregion 19 is formed in a <11{overscore (2)}0> direction of GaN.

Furthermore, a p-Al_(y)Ga_(1−y)N (Y=0.07) cladding layer 11 having athickness of 0.4 μm and a p-GaN contact layer 12 having a thickness of0.1 μm are formed in this order on the n-AlzGal-zN current blockinglayer 10 and the p-Al_(y)Ga_(1−y)N cladding layer 9.

Si is used as an n-type dopant, and Mg is used as a p-type dopant.Examples of a method of growing each layer include MOCVD (Metal OrganicChemical Vapor Deposition) or HVPE (Hydride Vapor Phase Epitaxy).

In this case, dislocations extend in the vertical direction from the GaNlayer 3 to the p-GaN contact layer 12 in a region where the SiO₂ film 4does not exist. No dislocations exist from the n-GaN layer 5 to thep-GaN contact layer 12 on the SiO₂ film 4.

As shown in FIG. 2, the region, from the p-GaN contact layer 12 to then-GaN layer 5, where the SiO₂ film 4 does not exist are removed by dryetching such as RIE or RIBE. Consequently, a GaN based semiconductorlayer 18 having no dislocations remains on the GaN layer 3.

Furthermore, in order to activate the p-type dopant, a p electrode 13composed of Ni having a thickness of 5000 Å, Pt having a thickness of100 Å, and Au having a thickness of 1 Am is formed on the p-GaN contactlayer 12, as shown in FIG. 3, after annealing at a temperature over 600°C., for example, at 800° C. for 30 minutes.

As shown in FIG. 4, an n-GaAs substrate 14 of 100 μm thickness havingohmic electrodes 15 a and 15 b formed on its top and reverse surfaces of(001) planes is prepared. The top surface of the p electrode 13 formedon the GaN based semiconductor layer 18 on the sapphire substrate 1 isjoined by thermocompression (bonding) or fusion to the ohmic electrode15 a on the n-GaAs substrate 14.

When the thermocompression is used, it is desirable that the surface ofthe p electrode 13 and the surface of the ohmic electrode 15 a arecovered with Au in a state immediately after evaporation. When thefusion is used, it is desirable that an Au—Sn film having a thickness ofapproximately 3 μm is formed on the n-GaAs substrate 14.

In joining the p electrode 13 and the ohmic electrode 15 a, therespective crystal orientations of the GaAs substrate 14 and the GaNbased semiconductor layer 18 are matched with each other such that then-GaAs substrate 14 and the GaN based semiconductor layer 18 have arelationship shown in FIG. 8.

In FIG. 8, the GaN based semiconductor layer 18 is formed on a (001)plane of the n-GaAs substrate 14. A striped current injection region 19corresponds to a region between the n-Al_(z)Ga_(1−z)N current blockinglayers 10 shown in FIG. 4, and a light emitting portion 20 is formed inthe current injection region 19. The current injection region 19 isprovided in a <1{overscore (1)}00> direction. In this case, the GaNbased semiconductor layer 18 is joined to the n-GaAs substrate 14 suchthat the current injection region 19 in the GaN based semiconductorlayer 18 is parallel to a <110> direction or a <1{overscore (1)}0>direction of the n-GaAs substrate 14.

The sapphire substrate 1 and the n-GaAs substrate 14 shown in FIG. 4 arethen dipped in a raw hydrof luoric acid, to remove the SiO₂ film 4. Thesapphire substrate 1 and the AlGaN buffer layer 2 and the GaN layer 3,having the dislocations, on the sapphire substrate 1 are removed fromthe GaN based semiconductor layer 18 on the n-GaAs substrate 14 bylift-off. Consequently, the GaN based semiconductor layer 18 having nodislocations remains on the n-GaAs substrate 14, as shown in FIG. 5. Inthis case, it is desirable that a raw hydrofluoric acid containing asurface active agent is used such that side etching of the SiO₂ film 4easily progresses.

Finally, as shown in FIG. 6a, an SiO₂ film 16 for preventing short isformed on the top surface and side surfaces of the GaN basedsemiconductor layer 18 and the surface of the ohmic electrode 15 aexcept in a region at the center of the n-GaN layer 5, and an nelectrode 17 composed of Ti having a thickness of 100 Å and Al having athickness of 2000 Å is formed on the n-GaN layer 5 and the SiO₂ film 16.

Thereafter, a pair of cavity facets is formed by a cleavage method. Inthis case, a {1{overscore (1)}00} plane of the GaN based semiconductorlayer 18 and a {110} plane or a {1{overscore (1)}0} plane of the n-GaAssubstrate 14 are cleavage planes, as shown in FIG. 8.

In the semiconductor laser device according to the present embodiment,the p electrode 13 and the n electrode 17 are respectively formed on thereverse surface and the surface of the GaN based semiconductor layer 18.Accordingly, a current path between the p electrode 13 and the nelectrode 17 is shortened. Further, few dislocations exist in the GaNbased semiconductor layer 18. Consequently, a low-voltage operation anda low-current operation are possible.

The cleavage direction of the GaN based semiconductor layer 18 and thecleavage direction of the n-GaAs substrate 14 can be matched with eachother. Accordingly, the cavity facets can be easily formed by thecleavage method.

Although in the present embodiment, description was made of a case wherethe present invention is applied to the semiconductor laser device, thepresent invention is also applicable to light emitting semiconductordevices such as a light emitting diode and the other semiconductordevices.

FIGS. 9 and 10 are schematic sectional views showing the steps of amethod of forming a GaN based semiconductor layer in a second embodimentof the present invention.

The top surface of a sapphire substrate 21 shown in FIG. 9(a) has a(0001) plane (c face). As shown in FIG. 9(b), an AlGaN buffer layer 22and an updoped GaN layer 23 are grown in this order on the (0001) planeof the sapphire substrate 21. Dislocations 37 extending in the verticaldirection exist in the GaN layer 23.

As shown in FIG. 9(c), the GaN layer 23 is then etched by RIE usingstriped masks 29 composed of Ni, to form a striped irregular pattern onthe surface of the GaN layer 23. The respective widths D of a recess anda projection in the irregular pattern shall be 5 μm, for example.

After the striped masks 29 are removed, an SiO₂ film 30 is then formedon the GaN layer 23, as shown in FIG. 9(d).

As shown in FIG. 10(e), the SiO₂ film 30 formed on side surfaces of theirregular pattern on the GaN layer 23 is then removed by etching.

Thereafter, as shown in FIG. 10(f), a GaN layer 24 is regrown. At thistime, the underlying GaN layer 23 is exposed only to the side surfacesof the irregular pattern. When the regrowth of the GaN layer 24 isstarted, theref ore, the GaN layer 24 Is not grown in the verticaldirection and is grown only In the lateral direction. The dislocations37 in the underlying GaN layer 23 are not propagated to the GaN layer 24which is grown in the lateral direction on the SiO₂ filMS 30.

As shown in FIG. 10(g), as the GaN layer 24 is regrown, the SiO₂ films30 on the lower step of the Irregular pattern are filled in with the GaNlayer 24. The GaN layer 2 4 Is grown in the vertical direction.

Thereafter, as shown in FIG. 10(h), the GaN layer 24 is grown in thelateral direction as well as the vertical direction on the SiO₂ films 30on the upper step of the irregular pattern. Accordingly, the surface ofthe GaN layer 24 is flattened. Consequently, the GaN layer 24 of highquality having no dislocations is formed on the SiO₂ films 30 on theirregular pattern.

In order to flatten the surface of the GaN layer 24 which is regrown,the GaN layer 24 must have a certain degree of thickness. The thicknessrequired to flatten the surface of the GaN layer 24 differs depending onthe growth conditions such as the width of the irregular pattern on theunderlying GaN layer 23 and the substrate temperature at the time of thegrowth of the GaN layer 24. For example, when the respective widths ofthe recess and the projection in the irregular pattern are approximately5 μm, the thickness of the GaN layer 24 must be approximately 10 to 20μm.

GaN is easily grown in a <1{overscore (1)}00> direction. In order thatthe GaN layer 24 is easily grown in the lateral direction in the stepsshown in FIGS. 10(f), 10(g), and 10(h), therefore, it is desirable thatthe striped masks 29 composed of Ni are formed in a <11{overscore (2)}0>direction perpendicular to the <1{overscore (1)}00> direction of the GaNlayer 23 in the step shown in FIG. 9(c).

In order that the surface of the GaN layer 24 which is regrown in thestep shown in FIG. 10(h) is easily flattened, the mask width of thestriped masks 29 composed of Ni and the window width of the stripedmasks 29 (the width of a region where no Ni exists) which are used inthe step shown in FIG. 9(c) are preferably as small as not more than 10μm, and more preferably 1 to 5 μm.

Furthermore, it is preferable that a cross-sectional shape of theprojection in the irregular pattern on the underlying GaN layer 23 is arectangular shape or a reversed mesa shape (reversed trapezoidal shape)having vertical side surfaces rather than a mesa shape (trapezoidalshape).

As shown in FIG. 11, the cross sectional shape of the irregular patternon the GaN layer 23 is a reversed mesa shape, and the Si0 ₂ films 30 areformed on the GaN layer 23 by a deposition method, such as electron beamevaporation, inferior in step coverage, thereby making it possible toprevent the SiO₂ films 30 from being deposited on the side surfaces ofthe irregular pattern. Consequently, it is possible to omit the step ofremoving the SiO₂ films 30 on the side surfaces of the irregular patternby etching.

FIGS. 12(a) and 12(b) are schematic sectional views showing a method offorming the irregular pattern in a reversed mesa shape on the surface ofthe underlying GaN layer 23.

As shown in FIG. 12(a), the striped masks 29 composed of Ni are formedon the GaN layer 23, and the sapphire substrate 21 is then inclined andis mounted on an etching apparatus at the time of dry etching. In thisstate, the GaN layer 23 is etched by dry etching such as RIE.

As shown in FIG. 12(b), the sapphire substrate 21 is inclined in thereverse direction. In this state, the GaN layer 23 is etched by dryetching such as RIE. The irregular pattern in the reversed mesa shapecan be thus formed on the surface of the GaN layer 23.

According to the method of fabricating the GaN based semiconductor layerin the present embodiment, it is possible to grow, even if the sapphiresubstrate 21 which differs in lattice constant from GaN, the GaN layer24 of high quality having no dislocations on the entire surface of thesapphire substrate 21.

When a light emitting semiconductor device such as a light emittingdiode or a semiconductor laser device composed of a GaN basedsemiconductor layer is fabricated on the GaN layer 24, it is possible toimprove light output power-current characteristic and reliability.

Although in the example shown in FIGS. 9 and 10, the sapphire substrate21 is used as an insulating substrate, the sapphire substrate 21 can bealso replaced with another insulating substrate such as an SiC substrateor a spinel (MgAl₂O₄) substrate.

FIG. 13 is a schematic sectional view showing an example of asemiconductor laser device fabricated on the GaN layer formed by themethod shown in FIGS. 9 and 10.

In FIG. 13, an n-GaN layer 25, an n-InGaN crack preventing layer 26, ann-AlGaN cladding layer 27, an MQW active layer 28, and a p-AlGaNcladding layer 29 are formed in this order on a GaN layer 24 formed bythe method shown in FIGS. 9 and 10. An n-AlGaN current blocking layer 31is formed except in striped regions on the p-AlGaN cladding layer 29. Ap-AlGaN cladding layer 32 and a p-GaN contact layer 33 are formed inthis order on the p-AlGaN cladding layer 29 and the n-AlGaN currentblocking layer 31. A partial region from the p-GaN contact layer 33 tothe n-GaN layer 25 is removed by etching. A p electrode 34 is formed onthe p-GaN contact layer 33, and an n electrode 35 is formed on theexposed surface of the n-GaN layer 25.

In the semiconductor laser device shown in FIG. 13, a GaN basedsemiconductor layer 36 from the n-GaN layer 25 to the p-GaN contactlayer 33 is formed on the GaN layer 24 having no dislocations.Accordingly, few dislocations exist in the GaN based semiconductor layer36. Consequently, a semiconductor laser device capable of a low-currentoperation and a low-voltage operation is obtained.

Although in the present embodiment, description was made of a case wherethe method of forming the nitride based semiconductor layer according tothe present invention is applied to the semiconductor laser device, themethod of forming the nitride based semiconductor layer according to thepresent invention is also applicable to light emitting semiconductordevices such as a light emitting diode and the other semiconductordevices.

Although in the example shown in FIGS. 9 and 10, the GaN layer 23 isetched such that it remains on the bottom surfaces of the recesses informing the striped irregular pattern on the surface of the GaN layer23, the GaN layer 23 may be etched until the sapphire substrate 21 isexposed to the bottom surfaces of the recesses, as shown in FIGS. 14 and15, in forming the irregular pattern on the surface of the GaN layer 23.

As shown in FIGS. 14(a) and 14(b), an AlGaN buffer layer 22 and anundoped GaN layer 23 are successively grown on a (0001) plane of asapphire substrate 21, as in FIGS. 19(a) and 19(b). The thickness of theGaN layer 23 is approximately 0.5 μm to 5 μm, for example. Also in thiscase, dislocations 37 extending in the vertical direction exist in theGaN layer 23.

An SiO₂ film is then formed on the entire surface of the GaN layer 23,and striped masks composed of photoresist are formed on the SiO₂ film.The SiO₂ film is etched using a hydrofluoric acid, to form striped SiO₂films 30, as shown in FIGS. 14(c).

Thereafter, as shown in FIG. 14(d), the GaN layer 23 and the AlGaNbuffer layer 22 are etched until the sapphire substrate 21 is exposedusing the SiO₂ films 30 as masks and by RIE using chlorine gas, to forma striped irregular pattern on the surface of the GaN layer 23. Therespective widths D of a recess and a projection in the irregularpattern shall be 5 μm, for example.

Thereafter, as shown in FIG. 15(e), the GaN layer 24 is regrown. At thistime, the underlying GaN layer 23 is exposed only to the side surfacesof the irregular pattern. When the regrowth of the GaN layer 24 isstarted, therefore, the GaN layer 24 is not grown in the verticaldirection and is grown only in the lateral direction. No dislocationsexist in the GaN layer 24 which is grown in the lateral direction on thesapphire substrate 21.

As shown in FIG. 15(f), as the GaN layer 24 is regrown, the recesses inthe irregular pattern are filled in with the GaN layer 24. The GaN layer24 is grown in the vertical direction.

Thereafter, as shown in FIG. 15(g), the GaN layer 24 is grown in thelateral direction as well as the vertical direction on the SiO₂ films 30on the top surfaces of the projections in the irregular pattern.Accordingly, the surface of the GaN layer 24 is flattened. Consequently,the GaN layer 24 of high quality having no dislocations is formed on theSiO₂ films 30 on the irregular pattern and the sapphire substrate 21.

As in the example shown in FIG. 9 and 10, in order to flatten thesurface of the GaN layer 24 which is regrown, the GaN layer 24 must havea certain degree of thickness. The thickness required to flatten thesurface of the GaN layer 24 differs depending on the growth conditionssuch as the width of the irregular pattern on the underlying GaN layer23 and the substrate temperature at the time of the growth of the GaNlayer 24. For example, when the respective widths of the recess and theprojection in the irregular pattern are 5 μm, the thickness of the GaNlayer 24 must be approximately 10 to 20 μm.

GaN is easily grown in a <1{overscore (1)}00> direction. In order thatthe GaN layer 24 is easily grown in the lateral direction in the stepsshown in FIGS. 15(e), 15(f), and 15(g), therefore, it is desirable thatthe striped SiO₂ films 30 are formed in a <11{overscore (2)}0> directionperpendicular to the <1{overscore (1)}00> direction of the GaN layer 23in the step shown in FIG. 4(c).

Furthermore, in order that the surface of the GaN layer 24 which isregrown in the step shown in FIG. 15(g) is easily flattened, the widthof each of the striped SiO₂ films 30 formed in the step shown in FIG.14(c) and the window width of the striped SiO₂ films 30 (the width of aregion where the SiO₂ films do not exist) are preferably as small as notmore than 10 μm, and more preferably not more than 1 to 5 μm.

It is preferable that a cross-sectional shape of the projection in theirregular pattern on the GaN layer 23 is a rectangular shape or areversed mesa shape having vertical side surfaces rather than a mesashape, as in the example shown in FIGS. 9 and 10.

Although in the example shown in FIG. 14 and 15, the sapphire substrate21 is used as an insulating substrate, the sapphire substrate 21 may bealso replaced with another insulating substrate such as a spinelsubstrate.

FIGS. 16 and 17 are schematic sectional views showing the steps of amethod of fabricating a semiconductor laser device in a third embodimentof the present invention.

As shown in FIG. 16(a), an AlGaN buffer layer 42 having a thickness of30 Å, an undoped GaN layer 43 having a thickness of 2 μm, and an Sidoped n-GaN layer 44 a having a thickness of 3 μm are grown in thisorder on a (0001) plane of a sapphire substrate 41.

As shown in FIG. 16(b), an SiO₂ film having a thickness of approximately1000 Å is then formed on the n-GaN layer 44a. The SiO₂ film in a regionexcluding a light emitting portion is then removed by etching, and theSiO₂ film in a region corresponding to the light emitting portion ispatterned in a stripe shape, to form a plurality of striped SiO₂ films45. In this case, the striped SiO₂ films 45 are formed in a <1{overscore(1)}20> direction of the n-GaN layer 44 a such that GaN is easily grownin the lateral direction in the subsequent step.

The width of each of the striped SiO₂ films 45 is approximately 0.5 ILm,and a pitch between the striped SiO₂ films 45 is approximately 1 μm. Inorder to realize fundamental transverse mode lasing, the width W1 of thelight emitting portion is preferably about 2 to 5 μm, and the number ofthe striped SiO₂ films 45 must be approximately 3 to 5.

Thereafter, as shown in FIG. 16(c), an Si doped n-GaN layer 44 b havinga thickness of 5 μm, an Si doped n-InGaN crack preventing layer 46having a thickness of 0.1 μm, and an Si doped n-AlGaN cladding layer 47having a thickness of 1 μm are grown in this order on the n-GaN layer 44a. Further, an MQW active layer 48 having the structure shown in FIG. 7,an Mg doped p-AlGaN cladding layer 49 having a thickness of 1 μm, and anMg doped p-GaN contact layer 50 having a thickness of 0.1 μm are grownin this order on the n-AlGaN cladding layer 47.

In this case, in a region where the striped SiO₂ films 45 exist, GaN isgrown in the vertical direction from the underlying n-GaN layer 44 abetween the striped SiO₂ films 45, and GaN is then grown in the lateraldirection on the striped SiO₂ films 45. On the other hand, in a regionwhere the striped SiO₂ films 45 do not exist, GaN is grown in thevertical direction from the underlying n-GaN layer 44 a. Consequently, adifference substantially arises in the growth rate of GaN between theregion where the striped SiO₂ films 45 exist and the region where thestriped SiO₂ films 45 do not exist. That is, in the region where thestriped SiO₂ films 45 exist, the growth rate of GaN is substantiallylower, as compared with that in the region where the striped SiO₂ films45 do not exist. The difference in the growth rate is continued untilthe striped SiO₂ films 45 are completely filled in with GaN so that thegrowth in the lateral direction is completed.

As a result, the surface of the n-GaN layer 44 b is concavely formed.Further, the n-InGaN crack preventing layer 46, the n-AlGaN claddinglayer 47, the MQW active layer 48, the p-AlGaN cladding layer 49, andthe p-GaN contact layer 50 are concavely formed. A concave part of theMQW active layer 48 is a light emitting portion which is an activeregion of the device. Further, few crystal defects exist in a GaN basedsemiconductor layer 56 on the region where the striped SiO₂ films 45exist.

Thereafter, as shown in FIG. 17(d), a partial region from the p-GaNcontact layer 50 to the n-GaN layer 44 b is removed by etching, toexpose the n-GaN layer 44 a.

Furthermore, as shown in FIG. 17(e), an SiO₂ film 51 is formed on thetop surface and side surfaces of the p-GaN contact layer 50 and the topsurface of the n-GaN layer 44 a in order to narrow a current and protectan exposed portion of a pn junction except in a region above the lightemitting portion and an electrode forming region of the n-GaN layer 44a.

Finally, as shown in FIG. 17(f), a p electrode 52 is formed on theexposed surface of the p-GaN contact layer 50, and an n electrode 53 isformed on the exposed surface of the n-GaN layer 44 a.

In order that a current is not uselessly injected into the lightemitting portion, it is preferable that the window width W2 of the SiO₂film 51 to be a current injection region is made equal to or slightlysmaller than the width W1 of the light emitting portion.

In a method of fabricating the semiconductor laser device according tothe present embodiment, the crystallizability of a GaN basedsemiconductor layer 56 on the region where the striped SiO₂ films 45exist, and the MQW active layer 48 in the light emitting portion isconcavely formed by a difference in the growth rate which arises duringthe growth in the lateral direction of GaN.

Consequently, a difference in the index of refraction appears in notonly the vertical direction but also the horizontal direction of thesapphire substrate 41. As a result, it is possible to easily fabricate arefractive index guided structure by performing crystal growth twicewithout carrying out the etching step. Consequently, a refractive indexguided semiconductor laser device which is uniform in devicecharacteristics and is high in reproducibility is realized.

Although in the present embodiment, description was made of a case wherethe present invention is applied to the semiconductor laser device, thepresent invention is also applicable to light emitting semiconductordevices such as a light emitting diode and the other semiconductordevices.

Although in the third embodiment, description was made of thesemiconductor laser device using the sapphire substrate 41, it ispossible to use substrates such as an SiC substrate and a spinel(MgAl₂O₄) substrate in place of the sapphire substrate 41.

Although in the above-mentioned first, second and third embodiments, theSiO₂ films 4, 30, and 45 are used as insulating films for carrying outepitaxial lateral overgrowth, the SiO₂ film may be replaced with otherinsulating films such as an Al₂O₃ film and a TiO₂ film.

Although in the above-mentioned first, second and third embodiments, thenitride based semiconductor layer contains Ga, Al, and In, the nitridebased semiconductor layer may contain boron (B).

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

What is claimed is:
 1. A method of fabricating a semiconductor device,comprising the steps of: forming a first nitride based semiconductorlayer containing at least one of boron, gallium, aluminum and indium onan insulating substrate; forming an insulating film in a region on saidfirst nitride based semiconductor layer; forming a second nitride basedsemiconductor layer containing at least one of boron, gallium, aluminumand indium using epitaxial lateral overgrowth on said first nitridebased semiconductor layer and said insulating film; removing said secondnitride based semiconductor layer except in a region on said insulatingfilm; joining the top surface of said second nitride based semiconductorlayer on said insulating film to one surface of a gallium arsenidesubstrate through a first electrode layer; removing said insulatingfilm, to remove said insulating substrate and said nitride basedsemiconductor layer from said second nitride based semiconductor layer;and forming a second electrode layer on said second nitride basedsemiconductor layer.
 2. The method according to claim 1, wherein thestep of forming said second nitride based semiconductor layer comprisesthe step of forming an active layer.
 3. The method according to claim 2,wherein the step of forming said second nitride based semiconductorlayer further comprises the step of forming a striped current injectionregion for injecting a current into said active layer along a<1{overscore (1)}00> direction of said second nitride basedsemiconductor layer, and the step of joining the top surface of saidsecond nitride based semiconductor layer to one surface of the galliumarsenide substrate through the first electrode layer comprises the stepof matching the <1{overscore (1)}00> direction of said second nitridebased semiconductor layer with a <110> direction or a <1{overscore(1)}0> direction of said gallium arsenide substrate, said fabricatingmethod further comprising the step of forming a pair of cavity facets bycleavage along a {110} plane or a {1{overscore (1)}0} plane of saidgallium arsenide substrate and a {1{overscore (1)}00} plane of saidsecond nitride based semiconductor layer.
 4. A method of forming anitride based semiconductor layer, comprising the steps of: forming afirst nitride based semiconductor layer containing at least one ofboron, gallium, aluminum and indium on an insulating substrate; forminga pattern including a recess having a bottom surface formed of aninsulator and a projection having a top surface formed of an insulatorin said first nitride based semiconductor layer so that said firstnitride based semiconductor layer is exposed on the side surface betweensaid bottom surface of said recess and said top surface of saidprojection; and forming a second nitride based semiconductor layercontaining at least one of boron, gallium, aluminum and indium on saidinsulators by growth from said first nitride based semiconductor layerexposed on the side surface of said pattern using epitaxial lateralovergrowth.
 5. The method according to claim 4, wherein the step offorming said pattern comprises the steps of forming said pattern suchthat said first nitride based semiconductor layer is exposed to thebottom surface of said recess, and forming an insulating film as saidinsulator on the bottom surface of the recess of said pattern andforming an insulating film as said insulator on the top surface of theprojection of said pattern.
 6. The method according to claim 4, whereinthe step of forming said pattern comprises the step of forming aninsulating film as said insulator in a region on said first nitridebased semiconductor layer where the projection of said pattern is to beformed and removing said first nitride based semiconductor such thatsaid insulating substrate is exposed as said insulator except in aregion on said insulating film.
 7. The method according to claim 4,wherein said irregular pattern has a striped recess and a stripedprojection which extend along a <11{overscore (2)}0> direction of saidfirst nitride based semiconductor layer.
 8. The method according toclaim 4, wherein a cross-sectional shape of the projection of saidpattern is a rectangular shape or a reversed mesa shape having verticalside surfaces.
 9. A method of fabricating a semiconductor device,comprising the steps of: forming a first nitride based semiconductorlayer containing at least one of boron, gallium, aluminum and indium onan insulating substrate, forming a plurality of striped insulating filmsa distance away from each other on said first nitride basedsemiconductor layer; and forming a second nitride based semiconductorlayer containing at least one of boron, gallium, aluminum and indiumusing epitaxial lateral overgrowth on said first nitride basedsemiconductor layer and said plurality of striped insulating films, toform an active region of the device above said plurality of stripedinsulating films.